Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and a rotor. The rotor typically includes a rotatable hub having one or more rotor blades attached thereto. A pitch bearing is typically configured operably between the hub and a blade root of the rotor blade to allow for rotation about a pitch axis. The rotor blades capture kinetic energy of wind using known airfoil principles. The rotor blades transmit the kinetic energy in the form of rotational energy so as to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.
At low wind speeds, there is insufficient torque exerted by the wind on the rotor blades to make them rotate. However, as the wind speed increases, the rotor of the wind turbine begins to rotate and generate electrical power. The wind speed at which the wind turbine first starts to rotate and generate power is generally referred to as the cut-in wind speed. As the wind speed rises above the cut-in wind speed, the level of electrical power rises rapidly until the power output reaches the limit that the electrical generator of the wind turbine is capable of, which is generally referred to as the rated power output. Similarly, the wind speed at which the rated power is reached is generally referred to as the rated wind speed. At wind speeds above the rated wind speed, the wind turbine is designed to limit the power output to the rated power. To avoid damage to the wind turbine, a braking system is typically employed when the wind speed reaches a cut-out wind speed. Thus, for conventional operation, the rated wind speed is a constant value. In other words, when the rotor reaches the rated power from an increase in wind speed, it maintains that value as winds continue to increase.
Typically, the wind turbine operates such that it reaches a rated rotor speed at a wind speed at or below the rated wind speed. In the upper partial load region of operation, defined on a torque-speed curve as the portion at rated speed and increasing torque to rated power, the wind turbine experiences lower performance due to operating away from its optimal tip speed ratio (TSR). Such operation introduces the potential for reduced aerodynamic efficiency and the need to mitigate that potential. Increasing the rotor speed allows the wind turbine to maintain optimum TSR operation up to a higher wind speed; however, the system is electrically, mechanically, and/or thermally limited such that it cannot maintain the higher generator speed at rated power levels.
Accordingly, a system and method that addresses the aforementioned problems would be welcomed in the technology. For example, a system and method that incorporates a variable rated speed set point in partial load operation of the wind turbine would be advantageous.